In fill level measuring technology there are numerous applications in which a high temperature or a high pressure is present. The process temperature may, for example, be up to 400° C. The pressure can have a value of up to 160 bar or more.
To make it possible in these applications to read the fill level with radar sensors, the process connections, in other words the antennae, may have to be constructed such that they withstand this temperature or this pressure without suffering any damage. To this effect, materials such as ceramics, glass or special steel are usually used.
Plastics that are used in known sensors as antenna fillings or seals are above all unable to withstand the high temperatures. These plastics are, for example, polytetrafluoroethylene (PTFE) or polyester ester ketone (PEEK).
If these plastics are to be replaced by ceramics or glass, then due to the high relative permittivities ∈r of the ceramics and of the glass this may not be achieved without further ado especially in the case of higher transmission frequencies of the radar sensor.
On the one hand the dimensions will be very small, and on the other hand the high-frequency behaviour is negatively affected by the large difference between ∈r and the surrounding air.
At frequencies around 6 GHz this may already cause problems and may significantly reduce the sensitivity of a sensor in particular at short range. FIG. 1 shows such a sensor.
A further option of process separation consists of the use of a ceramic plate as a window in a hollow conductor. In this arrangement a plate with a thickness that corresponds to half the wavelength of the transmission frequency (or integral multiples thereof) is inserted as a “window” into the hollow conductor. This is shown in FIG. 2. However, the reflection resulting from the high ∈r of the plate is so high that in this arrangement too the sensitivity at short range can be significantly reduced by multiple reflections.